This invention relates to blends of propylene/.alpha.-olefin copolymers with isotactic polypropylene which cover a wide range of products having properties ranging from those of elastomers to rigid plastics. In these blends, isotactic polypropylene is cocrystallizable with the propylene/.alpha.-olefin copolymers.
U.S. Pat. No. 3,888,949 to Chi-Kai Shih describes blends of stereoregular propylene/.alpha.-olefin copolymers with polypropylene of similar stereoregularity, which have improved injection molding characteristics. The patentee speculates that the desirable properties of his blends are due to the cocrystallization phenomenon. The propylene/.alpha.-olefin copolymer has a high crystallinity number, within the range of 40-70.
U.S. Pat. No. 3,882,197 to Fritz et al. describes blends of stereoregular propylene/.alpha.-olefin copolymers, stereoregular polypropylene, and ethylene copolymer rubbers. These blends all have improved injection molding characteristics but are softer than the blends of U.S. Pat. No. 3,888,949, which inherently are hard materials. It is believed that the propylene/.alpha.-olefin copolymers used in both U.S. patents and earlier disclosed in abandoned application of Fritz et al. Ser. No. 281,942 were fairly heterogenous, that is, contained a rather broad mixture of copolymers of different propylene/.alpha.-olefin ratios. Because of this, a rather high crystallinity number was required to impart to the blends sufficient strength. These elastoplastic compositions had a rather high hardness (Shore A higher than 90) and thus were not suitable for applications where soft, elastomeric properties were required, for example, molded rubber shoe soles.
It would be very desirable to be able to produce blends of isotactic polypropylene with lower crystallinity propylene/.alpha.-olefin copolymers which would span a broader range of hardness, for example, from Shore A of about 70 to more than 90, yet would not require a third blend component. However the polymerization catalysts used to make the copolymers of Fritz et al. application Ser. No. 281,942 were incapable of producing satisfactory copolymers for this purpose.